evolution of the h2o maser emission in g10.6-0.4

ISSN 1063-7729, Astronomy Reports, 2006, Vol. 50, No. 8, pp. 638–646. c© Pleiades Publishing, Inc., 2006.Original Russian Text c© E.E. Lekht, N.A. Silant’ev, V.V. Krasnov, V.A. Munitsyn, 2006, published in Astronomicheskiı Zhurnal, 2006, Vol. 83, No. 8, pp. 716–725.

Evolution of the H2O Maser Emission in G10.6−0.4

E. E. Lekht1, 2, N. A. Silant’ev1, 3, V. V. Krasnov4, and V. A. Munitsyn5

1Instituto Nacional de Astrofısica, Optica y Electronica, Luis Enrique Erro No. 1,Apdo Postal 51 y 216, 72840 Tonantzintla, Puebla, Mexico

2Sternberg Astronomical Institute, Universitetskiı pr. 13, Moscow, 119992 Russia3Main Astronomical Observatory, Russian Academy of Sciences, Pulkovo, St. Petersburg, 196140 Russia

4Astro Space Center, Lebedev Physical Institute, Russian Academy of Sciences,Profsoyuznaya ul. 84/32, Moscow, 117997 Russia

5Space Research Institute, Russian Academy of Sciences,Profsoyuznaya ul. 84/32, Moscow, 117810 Russia

Received December 20, 2005; in final form, February 6, 2006

Abstract—The water-vapor maser emission in the source G10.6−0.4 associated with an active star-forming region (OB star cluster) is analyzed. The maser was monitored from 1981–2004 usingthe 22-meter radio telescope of the Pushchino Radio Astronomy Observatory. Statistical processing ofthe results revealed the presence of structural formations on various scales. The individual H2O maserfeatures may form ordered structures with velocity (VLSR) gradients, localized in separate clusters of maserfeatures. The statistical variations of the VLSR values for the maser components may be due to the accretionof material onto the OB star cluster in G10.6−0.4 together with the rotation of the molecular cloud core. Amodel with a rotating, nonuniform condensation of accreted material in the vicinity of the stellar cluster isproposed to explain the variations of the velocity centroid of the H2O spectra. The integrated flux variationsare explained well by a model in which the central source is an OB star cluster, possibly containing five tosix stars. An important role in the evolution of the maser emission, as well as of the source as a whole, maybe played by turbulent motions of the gas.

PACS numbers : 98.38.Er, 98.35.AcDOI: 10.1134/S106377290608004X

1. INTRODUCTION

VLBI studies and long-term monitoring of H2Omasers have shown that these sources have a com-posite structure with a large number of individualemission features (maser spots). Strong variability oftheir fluxes, radial velocities, and linewidths is ob-served. The emission features can be joined in sep-arate spectral and spatial groups. Structures suchas compact groups (see, e.g., [1, 2]), chains [3, 4],filaments, and others can be present. The distribu-tion of maser spots in a source and the evolution oftheir emission basically reflect the morphology of theregions within the source: Keplerian disks, bipolaroutflows, or HII regions in various configurations,such as shell-like and cometary forms [5].

An important role in the formation of structures inthe interstellar medium is played by turbulent motionson various scales. The hierarchical structure of thisturbulence is determined by the nonlinear couplingof all its components. Based on interferometric ob-servations in the 1.35 cm line, Gwinn [6] found the

spectrum of the large-scale (up to 1000 AU) super-sonic turbulent motions in the source W49N for atypical maser feature size of ∼1 AU. The observationsdemonstrate the presence of a turbulent mediumwith a power-law velocity spectrum on scales ofl = 1−300 AU. On scales l > 300 AU, maser fea-tures are ordered in arches oriented perpendicular tothe direction of the expansion. In contrast to this,the small-scale turbulence is associated with thestructures of individual maser spots. For instance,structures such as an inhomogeneous rotating vortexhave been detected [7, 8] and their main parametersdetermined: their characteristic size R0, velocity u0,and life time τ0.

Earlier, we considered the small-scale structureand parameters of structures associated with indi-vidual maser spots in the active star-forming regionG10.6−0.4 [9, 10]. In the current paper, we searchfor larger-scale structures in the hierarchy of complexmotions of the material in G10.6−0.4 based on ourmonitoring of the H2O maser from 1981–2004.

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EVOLUTION OF THE H2O MASER 639

–8

19841980 1988 1992 1996 2000 2004Years

–12

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ial v

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ity, k

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Fig. 1. Variations of VLSR for the main H2O components in G10.6−0.4. Sequences of points with similar VLSR are fitted withstraight lines.

2. STRUCTURE OF THE G10.6−0.4 REGION

The H2O maser source that is frequently calledW31(2) is located in the active star-forming regionG10.6−0.4, which has a complicated structure [11,12]. The region hosts a molecular core, which con-tains a cluster of forming massive OB stars. The coreradius is ∼0.4 pc. According to VLA NH3 line obser-vations, a radial-velocity gradient of ∼10 km s−1pc−1

[12] was detected in the core, suggesting that the coreis rotating. The core is located within a slowly rotatingenvelope with a radius of ≥1.2 pc and with a velocitygradient of about 1 km s−1pc−1 [12]. Parts of the corewith a redshift of ∼5 km/s relative to its velocitycentroid (−1.2 km/s [13]) detected in absorption arecollapsing towards the cluster of OB stars [12]. Theestimated core mass is 1200M� [14].

Sollins and Ho [15] found from VLA observationsin the continuum and in the NH3 (3, 3) inversion linea flattened, rotating, molecular accretion flow. How-ever, the accretion flow in G10−0.4 does not form anaccretion disk. The unusual structure of the regionsuggests a different mode of clustered massive starformation, different from accretion disks. Observa-tions in the H66α line [16] showed that ionized gas inthe ultracompact HII (UC HII) region in G10.6−0.4does not move outward, as in the classic models ofHII regions, but instead spirals toward the stars at

the center of the UC HII region. Sollins and Ho [15]concluded that the accretion flow is spherical at largedistances, whereas the core rotation flattens the gasaccretion flow at distances comparable to the size ofthe UC HII region.

According to Genzel and Downes [17], and Fazioet al. [18], the H2O maser is in close proximity to theUC HII region formed by the OB star cluster. Follow-up VLA observations of the H2O maser in G10.6−0.4with an angular resolution of 0.4′′ yielded accuratepositions of individual clusters of maser spots relativeto the ultracompact HII region [19]. The region ofthe strongest maser emission coincides with the NH3

gas position, and forms a linear structure ≈0.04 pcin size, directed perpendicular to the rotational axis ofthe molecular gas. However, the clusters themselveswere not resolved into individual emission features.

The H2O maser is probably not associated witha single massive star of the OB cluster, but withseveral, though the influence of one of them may dom-inate. Some maser spots could be associated withfragments in the cloud core, which is collapsing to-ward the OB star cluster [9]. Our 24-year monitoringof the H2O maser has shown that the variability ofthe integrated emission has a flare like character, andno long-period variability has been detected. How-ever, the variability of the velocity centroid (weighted-mean radial velocity) is fairly well approximated by a

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Number of jumps (‡)

(b)

00.4

4

0.8 1.2 1.6 2.0 2.40Interval length, years

Number of intervals

8

12

Fig. 2. Histogram of the distributions of (a) radial velocityjumps and (b) time intervals between adjacent emissionfeatures (fragments) for the H2O maser in G10.6−0.4(see text for details).

sine wave with a period of the order of 31–33 yearsand an amplitude of 1.1 km/s relative to the value−0.8 km/s [9], which nearly coincides with the centerof the H66α recombination line (−1.2 km/s) [13].

3. DATA

A complete catalog of our H2O spectra from1981–2004 was published by Lekht et al. [9, 10].The mean interval between observations was about1.6 months, with intervals of 0.75–1.5 months beingmost typical. As noted earlier, all the spectra werecorrected for absorption into the Earth’s atmosphere.

Figure 1 presents the drift of the main emissionpeaks in the H2O spectra from 1981–2004, i.e., theirtemporal evolution. The vertical arrows mark themaxima of two superflares that took place from 1985–1986 and 1998–1999.

To isolate individual features associated with thesmall-scale structure of the maser emission regionsat the level of individual maser spots, we fitted straightlines to sequences of points with sufficiently similarradial velocities. The observed velocity jumps betweenadjacent fragments, indicating that some emissionfeatures have decayed, while others have appeared,and the flaring features began to dominate the decay-ing features in these intervals. We disregarded smallvelocity jumps, since they could be related to the finerstructure in the maser spots. Furthermore, the effectof the measurement errors is stronger here than forlarge VLSR jumps, which can distort the histogram.The errors in the velocity measurements did not ex-ceed ±0.015 km/s. We did not use data for which thevelocity jumps between features were widely spacedin time, e.g., component 11.

Figure 2a shows the distribution of the numberof radial-velocity jumps (∆V ) between features withclose VLSR values. The jumps were counted usingintervals of 0.1 km/s. The distribution is not ran-dom. The main maximum is at 0.45 km/s. Note thatthe lifetimes of the main emission features are muchlonger than the intervals between consecutive ob-servational sessions. Thus, we can assume that theobserved distribution of VLSR jumps (Fig. 2a) is in noway related to the intervals between monitoring ob-servations, and can have some real physical meaning.

We also determined the time intervals between thecenters of adjacent features. The distribution of thenumber of intervals with a step of 0.1 year is shownin Fig. 2b. The main peak is near 0.9 year; there is aweaker one at ∼0.3 year, which is associated with afast velocity drift in some components. In most casesthis corresponds to epochs of strong flaring in theH2O maser.

For each time interval, there is a correspondingvalue of the velocity jump. The relationship betweenthese parameters is given in Fig. 3. In the upperpanel, the jump direction was taken into account,so that there are two dependences. The dashed linesshow the fitted linear relations. In the lower panel, themagnitudes of the velocity jumps are plotted together

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EVOLUTION OF THE H2O MASER 641

0–0.5 0.5 1.0 1.5 2.0 2.5Interval, years

0.5

0

1.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

(b)

(‡)Ju

mp

mag

nitu

de, k

m/s

Fig. 3. Magnitude of the VLSR jumps as a function of thetime interval.

with our fitted straight line. There is a slight tendencyfor the VLSR jumps to decrease with an increasingcomponent lifetime, irrespective of the direction of thevelocity change.

We plotted the distributions of the total number ofthe fragments using three different binning intervals:2, 1, and 0.5 km/s (Fig. 4). The histograms have beenapproximated with smoothed curves. We have fittedtwo Gaussians centered at −5.6 and 0.6 km/s to thedistribution in the upper figure. Narrowing the inter-val from 2 to 0.5 km/s allows us to see finer structurein the distribution; the fine structure components arenumbered.

In Fig. 5, sequentially appearing emission features(we call them “fragments”) with close velocities areconnected with curved (solid) lines. The large opencircles mark the positions of the maxima that werederived from the mean spectra for different time in-tervals. We split the data into intervals in accordancewith the maser’s activity cycles [10]. These basically

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Fig. 4. Histogram of the radial-velocity distribution of thetotal number of fragments measured in intervals of 2, 1,and 0.5 km/s.

served as reference points for fitting smooth (dashed)curves, which, for convenience, are labeled with thesame numbers as in Fig. 4. We introduced numerical-alphabetic designations for the detected groups.

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6a 7a 7b 7c7

7d

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11

Fig. 5. Same as in Fig. 1, but with ordered structures outlined (solid curves). The large open circles show the flux maxima inthe average spectra [10]. The smooth dashed curves show the main components 1–11 (see text for details).

4. DISCUSSION

The observed radial-velocity drifts reflect the dis-placement of the main emission peaks in the H2Ospectra. Each peak corresponds to emission from asingle feature or to the maximum of a superposition ofseveral features with close radial velocities (a group offeatures). There are both jump like and smooth veloc-ity variations. There are from one to four consecutiveVLSR jumps in one direction. A cluster of such frag-ments can be ordered, e.g., as elongated structuresdisplaying a gradient of VLSR. If the direction of thejumps changes, the cluster of fragments (group) hasno ordered structure.

Jump like changes in VLSR can be produced bythe consecutive excitation of maser features thatare located in the above-mentioned structures inG10.6−0.4 and whose presence is manifested duringthe flare activity of the maser. The variability ofthe integrated H2O flux of G10.6−0.4 had a flarecharacter with a mean period of ∼2 years [10]. This,together with the inhomogeneity of the medium,explains the prevalence of velocity jumps throughoutmonitoring. More or less smooth drifts can be due tothe actual nonuniform motion of a maser spot, e.g.,with accelerated motion during a flare.

Over a time interval of 1.5–2 years, there were, atfirst glance, random variations in the peak positions

of some components (1b, 3c, 4b, 5b, and 8c) within0.6–1.2 km/s. In fact, this variability had a more orless periodic character. We reduced these segmentsof the VLSR variability to the same scale in time andvelocity, superposed the main maxima, and deter-mined the time intervals for the secondary maxima foreach component. After determining the mean inter-val, we normalized, over the time scale, to the valueof the mean interval and fitted a smooth curve (notshown here). Its behavior can be explained if each ofthe above components contains two main fragments,which have a velocity difference of ≈0.6 ± 0.1 km/s,and appear alternately with an interval between theflux maxima of about 0.4 ± 0.1 year. The obtainedvalue ∆t ≈ 0.4 year coincides on the histogram ofFig. 2b with the maximum that corresponds to highmaser activity, and ∆V ≈ 0.6 km/s is close to themain maximum in the distribution of Fig. 2a. Thus,the additional data on five components reinforce thehistograms shown in the Fig. 2.

We consider three main scales in the hierarchicalstructure of the maser source.

1. Individual maser features (“spots”).

2. Ordered structures that include several maserspots (e.g., chains, filaments). Clusters of individualmaser spots are also possible.

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EVOLUTION OF THE H2O MASER 643

3. Large-scale structures, such as arches or frag-ments of shells.

Our subsequent analysis is devoted to structuresof the second and third types.

4.1. Ordered Structures

In addition to the histograms of Fig. 2, we presentsome statistics of the VLSR jumps. The number ofvelocity jumps in the direction of decreasing velocitywas 56, and the sum of all the jumps was about−31.8 km/s. In the direction of increasing veloc-ity, these parameters are 29 and 17.5 km/s, respec-tively. Thus, there was a dominant direction in theVLSR drift of the components. However, the meanmagnitudes of the jumps were fairly similar: −0.57and 0.61 km/s. We have for the jump time intervals0.88 and 0.93 years, respectively.

For components with >2 fragments, we found thefull velocity drift in one direction (∆Vc) and the timeintervals ∆Tc in which the drift was observed. Thescatter in these parameters was fairly large: 0.5–3.5 km/s for ∆Vc and 1.1–3.9 years for ∆Tc. The dis-tributions of components in ∆Vc and ∆Tc are shownin Fig. 6. The time intervals ≈1.3 and ≈2.2 years andvelocity drifts of 0.9–1.5 km/s are most typical, whilethe average values of these parameters over the entireensemble of individual fragments were 0.9 years and0.58 km/s, respectively.

Our analysis of the two peaks in the histogramin Fig. 6b showed that the components from 1982–1991 (1, i.e., associated with the first superflare) dom-inated in the first, while the components from 1992–2004 (2, the interval of the second superflare) dom-inated in the second. This is probably due to evo-lutionary processes, in particular, to changes in theturbulence mode and rate of accretion of material ontothe cluster of OB stars. The vertical bars in the upperpanel show the positions of the average ∆Vc for thecomponents of the second and first time intervals.

The sequential appearance of the emission ofthe features (fragments) with a smooth variation intheir radial velocities may testify to the presence ofa VLSR gradient in the region hosting these H2Omaser spots, which can form structures, such asinhomogeneous filaments.

The change in the direction of the velocity drift ofthe components (sequence of jumps or smooth driftin VLSR, Fig. 5) could be due to the repeated actionof the stellar wind during the following flare activityof the central source. The next wave of componentactivation arrives, and again excites the emission ofthe first fragment, so that there is a return to the stateclose to the initial one. This evolutionary pattern can

2

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, km/s

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T

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, yr

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(‡)

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Fig. 6. Histograms of the distributions of the number ofcomponents (a) in the total VLSR drift magnitude in onedirection and (b) in drift duration. The vertical bars showthe positions of the mean values of ∆Vc calculated from(1) 1981–1991 and (2) 1992–2004 (see text for details).

then be repeated. Of course, this is an idealized sce-nario. In reality, there could be large deviations due toevolutionary effects. Nevertheless, synchronous ve-locity variations in several components following thisscenario can be seen in Fig. 5.

4.2. Large-Scale StructureLet us list the results of our monitoring that can be

important in constructing a model for the source:

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—the radial velocity distribution of individual fea-tures (Fig. 4) contains two components (−5.6 and0.6 km/s), which have a finer structure;

—asymmetry in the number of VLSR jumping to-ward negative velocities is observed (Fig. 3);

—the distribution of the number of components intime intervals (histogram in Fig. 6) has two maxima;

—the position of the velocity centroid varies with aperiod of ∼31–33 years and an amplitude of 1.1 km/srelative to the value −0.8 km/s;

—there is a cyclical variability of the maser activitywith a mean period of about 2 years;

—a long-period component is not detected in themaser variability;

—the superflare of 1986 had comparable energy tothe superflares in Orion KL;

—a correlation between the emission in the mainpeaks of the H2O spectra, i.e., between various seg-ments of the spectra, is observed;

—there are anticorrelations between the fluxes ofsome emission features.

The existence of two main peaks in the histogramof Fig. 4 (−5.6 and 0.6 km/s) suggests that theclusters of maser features are spatially separated intotwo main groups. This matches the model of [15]for the composite region G10.6−0.4, in which theH2O maser is at the boundary of the UC HII regionlocated in the rotating core. In this model, there isalso a strong accretion of material into the central starcluster. This model probably involves radial motion ofthe maser spots. The fine-structure components ob-served by us (Fig. 4) can be identified with individualclusters of maser features according to the VLA mapof Sollins and Ho [15].

The predominance of negative jumps in the ra-dial velocities of the components (56, as opposed to29 jumps in a positive direction) can naturally beexplained by the difference in the number of maserspots in the regions in front of and behind the centerof the OB star cluster. The rotational motion of thedisk material away from the observer (observed inmolecular lines) results in negative velocity jumps inthe H2O components.

Since material is accreted from the disk onto thestar cluster, we can easily find that the velocity jumpsare negative and positive in front of and behind thecentral plane, respectively. This indicates that themain group of components (Fig. 4) is in front of thisplane. The maser spots within individual clusters [19]can be arranged along the line of sight, and so beat different distances from the center of the rotat-ing cloud core. This means that the velocity jumpsbetween fragments can have either sign throughout

the radial-velocity range of the H2O spectrum, as isobserved (Fig. 1).

The observed strongest maser emission of1986 matches the model for G10.6−0.4 well. TheH2O line profile of the 1986 superflare has twocomponents. They initially approached each other inthe spectrum, then separated slightly [10]. The mostsuitable model for explaining the observed variabilityof the flux, radial velocity, and the line shape has twomaser spots with close radial velocities moving in theplane of the sky superposed in the line of sight [20].Such a superposition could take place in the presenceof large-scale turbulent (vortical) motions involvinga group of maser spots or within a group of spots(Fig. 7).

Xie et al. [21] pointed out that the small size of theUC HII region and its relatively long lifetime cannotbe explained by the thermal motion of the surroundinggas alone. The presence of additional pressure, whichcould arise, e.g., due to a turbulent flow of gas at theboundary of the region, is required. Thus, turbulentmotions play an important role in the evolution of theG10.6−0.4 region.

An alternative possibility is that there is a dif-ference in the velocities of the accreted material inthe radial direction within a cluster of maser spots(Fig. 7). This is possible because the rate of accretionof material from the cloud core (which likely hoststhe maser spots) is fairly large, 5 km s−1pc−1. Here,a random projection of maser spots (that are in thesame cluster of emission features) in the line of sightis also possible [19].

Note that both superflares of the water maser inG10.6−0.4 (1986 and 1998) took place during a pe-riod of high maser activity. Given the difference intheir distances, the H2O maser in G10.6−0.4 is onlyslightly less powerful than the maser in Orion KL,whose flux during the superflares from 1984–1985and 1998–1999 reached 6.7 × 106 (e.g., [22]) and4.6 × 106 Jy [23], respectively. At the distance ofOrion KL, the flux from the feature that flared inG10.6−0.4 in 1986 would be no less than 0.9 ×106 Jy. Powerful maser flares have also occurred inother sources, such as Sgr B2. Thus, superflares ofH2O masers are not a unique phenomenon.

4.3. About the Model of the Region

One of main reasons that the center of the UC HIIregion hosts a cluster of massive stars could be thecharacter of the variability of the integrated flux (Fint)of the H2O maser. As noted in [9], a long-periodcomponent was not detected in Fint variations, whichwere confined to a narrow range of values within600 Jy km/s of the average level of about 900 Jy km/s

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Ionizationfront

Cloudcore

Shell

Clusters of maser

features

V

rot

Observer

OB starcluster

V

accr

UC HII

V

turb

Fig. 7. A sketch of the G10.6−0.4 region (fragment) with some clusters of H2O maser spots (dashed ovals).

(excluding the superflare). The presence of a starcluster has probably smeared long-term variability,which can have individual contributions from each ofthe stars.

The cyclic variability of the maser activity witha mean period of about two years [9, 10] is mostnaturally explained as superposition of the activity ofall the stars in the cluster. According to theoreticalstudies, the luminosity of a protostar varies due tothe non-stationary accretion of material (e.g., [24,25]). Accretion can fluctuate with a period from sev-eral years to tens of years. If we adopt a period of∼10−12 years, then, to obtain a roughly two-yearcycle, we must have five to six OB stars in the cluster.

The variability of the velocity centroid (Vc) witha period of ∼31−33 years is due to variations ofthe maser emission at radial velocities from −10 to7 km/s. According to Hoffman and Ho [19], thestrongest maser emission in G10.6−0.4 is associatedwith four main clusters of maser features, which arearranged along the boundary of the UC HII region.The small amplitude of the Vc variations (1.1 km/s)suggests that their origin could be a small spatialanisotropy of the UV radiation from the rotating clus-ter of OB stars, so that a “scanning” of the maximumUV radiation in the rotation plane takes places.

Since the spherical accretion flow flattens towardthe UC HII region in G10.6−0.4, it is natural tosuppose that the cluster of OB stars might be oblate.The existence of a rotating core hosting the UC HIIregion also leads to a second supposition: the stars inthe cluster may rotate about their common center of

mass. With such a rotation, small spatial (in the rota-tion plane) displacements of the anisotropy in the UVradiation of the stars are possible; these can result ina sequential increase in the emission of the clusters ofmaser spots. Another possible origin of the anisotropyvariation could be gas that is accreted onto the starcluster in spirals. It cannot be excluded that such gas-flow trajectories result in the gas possibly being ableto rotate more strongly as it moves toward the centerof the UC HII region. This rotating gas structure canbe inhomogeneous, giving rise to modulation of thestellar radiation.

Of course, observations over only one period of thevelocity-centroid variability cannot provide definiteanswers about the origin of this phenomenon.

5. CONCLUSIONS

Our statistical analysis of the evolution of the H2Omaser emission in G10.6−0.4 has revealed structuralformations on various scales.

The individual maser features (“fragments”) prob-ably form ordered structures, such as elongated, in-homogeneous “filaments” with radial-velocity gradi-ents. Such structures, which were detected thanks toour long-term monitoring, are associated with fourmain clusters of maser features [19]. Our lack ofability to resolve the clusters into individual emissionfeatures prevented us from identifying the detectedstructures more precisely.

The results of our statistical analysis of the VLSRvariations of individual H2O components are bestexplained by accretion of material onto an UC HII

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region, combined with weak rotation of the molecularcloud core [15].

The changes in the velocity centroid (Vc) with aperiod of 31–31 years can also be most naturallyattributed to the accretion of material onto an OB-star cluster. We suggest that such changes in Vc aredue to the rotation of an inhomogeneous flattenedclump of accreted gas, which can be located nearthe cluster of stars and can modulate their radiation.We also cannot exclude a small anisotropy of the UVradiation from the star cluster due to its rotation aboutits common center of mass.

The character of the integrated flux variations (theabsence of a long-period component and presence offlares with a mean period of about two years) leads usto adopt a model in which the central source is a starcluster containing five to six stars.

ACKNOWLEDGMENTS

This work on the RT-22 Radio Telescope wassupported by the Ministry of Education and Sci-ence of the Russian Federation (registration number01-10). The authors are grateful to the staff of thePushchino Radio Astronomy Observatory for theirconsiderable help with the observations of the H2Omaser in G10.6−0.4.

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Translated by G. Rudnitskii

ASTRONOMY REPORTS Vol. 50 No. 8 2006